In the plastic molding industry, for example, the effective blending of colorants with base plastic material has been a continuing problem. Although some blending occurs in the molding machine and extruder, it typically is inefficient and inadequate. To compensate for imperfect blending, it is frequently necessary to add additional colorant. This has many disadvantages as will be outlined hereinafter. Quite independent of the matter of color and blending, efficient mixing of the viscous plastic material, as it flows to the mold, can be important in achieving greater uniformity of material temperature. Thus, temperature across a stream of flowing plastic material may vary by as much as 50.degree. Fahrenheit, and this can be reduced or eliminated by efficient mixing of the plastic flow. This can have important advantages in reducing cycle time, providing greater uniformity of mold packing to reduce weight variations in the finished part, etc.
Because of the important advantages achievable from the blending of viscous plastic flow prior to discharge into a mold cavity, there have been a number of proposed arrangements for effecting so-called static mixing of the flowing matrial. In a static mixing procedure, the physical facility for effecting the mixing action is stationary or static, and the motion of the flowing plastic or other material is utilized to derive a mixing action.
The following United States Patents are illustrative of some of the known general types of static mixing units: Perdue U.S. Pat. No. 1,373,829, Boucher et al U.S. Pat. No. 3,128,794, Harder U.S. Pat. No. 3,195,865, Harder U.S. Pat. No. 3,404,869, Harder U.S. Pat. No. 3,583,678, Chisholm U.S. Pat. No. 3,652,061, Grout U.S. Pat. No. 3,704,006, Huber U.S. Pat No. 3,785,620. Insofar as I am aware, the static mixing devices reflected by these patents suffer from the disadvantage of either inefficient mixing performance or excessive pressure drop, or in some cases both. Thus, in those units where a relatively high degree of mixing efficiency is achieved, it is achieved at the cost of an excessively high pressure drop in the flowing material. This necessitates pumps and equipment of higher capacity or, more typically, simply restricts the operating capacity of the molding or other process equipment in an undesirable way. In the past, achievement of high efficiency mixing and the provision of low pressure drop flow through the static mixing unit have seemed to be mutually inconsistent objectives.
Pursuant to the present invention, a highly simplified and inexpensive static mixing unit is provided which not only achieves exceptionally high efficiency in its mixing and blending functions, but does so with an extremely low pressure drop. In achieving both high efficiency mixing and low pressure drop, the static mixing device of the present invention represents a highly significant advance in the art. Even more surprising, however, is that this significant advance is achieved in a device which is highly simplified in nature, economical to manufacture, and dependable in use.
According to another feature of the invention, a static mixing unit having the significant advantages mentioned above is provided, which can be easily removed for cleaning or replacement if it becomes clogged. In this respect, almost any plastic molding or other system carrying flowable materials will inevitably carry foreign or tramp material from time to time, which can lodge in and clog the static mixing unit. In many of the mixing units presently available on the market, the removal of lodged foreign matter from the interior of the mixing unit is difficult to impossible to accomplish.
Certain aspects of the invention are also directed to the improved method of blending flowable materials, as achieved in the use of the new mixing unit.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment and to the accompanying drawings.